CDU

The CDU (Computer Display Unit) is a Flight Management Computer that helps the pilot to controll the vertical and horizontal flight path. It is designed to decrease the workload of pilots in today’s cockpits.

For some years now, various flightsim related companies offer ready-to-use, plug-and-play units for cockpit builders. While quick and convenient, they are also expensive; and they are not what I had in mind: I wanted to build my own. But the CDU is a rather complicated piece of hardware, so this was not an easy task. This page explains the different phases of the building process.

1. Preparation and Design

The CDU consists of a large number of components. The first step in the process of building my CDU was to define and order all necessary parts. Once they arrived and I knew their physical dimensions, the design process can start. As always, I used Corel Draw for this. The design stage is where the final layout is decided, so it is not a good idea to rush things at this point: The more the building process advances, the more difficult it becomes to accomodate modifications.

On the left is the final result. By moving the cursor over the image you can see a wireframe version.

The form was mainly dictated by the large number of push buttons that had to be accomodated, and by the shape of the display.

Note the little holes around the buttons in the wireframe version. They will hold M2 spacer rods and are placed at a distance of 2,54 mm (or multiples) from each other. This is the standard distance of the holes on printed circuit boards (onto which the buttons are soldered).

2. Display

Since I did not find a small enough CTR monitor, I bought a 5,4″ LCD screen. They have a very chrisp display (not that the CDU display is very demanding in the first place), and they don’t take up much space. But they also have drawbacks: First, they are not symmetrical, and second, they look best when the viewline is perpendicular to the screen. If the viewing angel deviates too much from 90 degrees, the image looses contrast and colour. Kepp this in mind if you plan to use LCD screens in your cockpit.

The LCD display needs an analog-to-digital converter card, which in turn connects directly to the VGA output of any ordinary VGA graphics card. I decided to mount the converter card directly below the display, because the cables that came with the card were short.

I used threaded brass rods (M2) to both fix the display on the panel and to act as spacers between display and converter card. They have to be bent into shape because the mounting holes in display and converter card obviously do not coincide.

The converter card requires 12V DC at a clamp on the card. The respective cabel is routed to a an DC switch panel. From there I can individually switch the different electrical components in my cockpit on or off.

3. Wiring The Buttons

The CDU uses almost 70 keys. I did not have enough free buttons on my EPIC card, so I decided to use a hacked keyboard instead. Because of the display, the Project Magenta CDU programm runs on a dedicated PC anyway, so this was an obvious (and cheap) solution. For information on how to hack a keyboard, click the link.

This is the back of the keypad area. As you can see, I used different circuit boards for the mounting of the function keys, the alphabetic and the number keys. They are kept at a fixed distance from the surface of the panel with M2 threaded rods as spacers.

Not only does it look messy, it actually is! What can I say? It is best to use colour coded wire, and for the rest: just keep your wits together while soldering something like this!

Frequent error testing and connectivity checks are also very much recommended.

This photo shows the M2 spacer rods used to hold the circuit board at a fixed distance from the front plate. The pushbuttons are soldered to the board. Precision is of utmost importance, else the buttons will not fit well and might get stuck after being pressed.

4. The Case

The CDU panel is a bit wider than the center pedestal in my cockpit. Instead of rebuilding the center pedestal I decided to build a case for the CDU assembly. With the analog-to-digital converter card and all the wiring sticking out, some protection seemed like a good idea anyway. The case is made from plywood.

It also holds the panel for the Transponder, right below the CDU.

The PCB comes from the hacked keyboard. Connections between the PCB and the buttons are done with 2 DB25 connectors.